Diagenesis and sedimentary environment of Miocene series in Eboliang III area

This paper presents petrology, mineralogy, and elemental geochemistry research on diagenesis, formation environment, and source material of the Upper Ganchaigou Formation of clastic rocks in the Eboliang III structural belt on the northern margin of the Qaidam basin. Point-line contacts of particles and directional alignment of feldspars show relatively intense compaction. Clays have high content with mean up to 35.03 % of total cement content, mainly consisting of illite, chlorite, and mixed-layer illite–smectite. Carbonate cements contain large amounts of calcite and little dolomite. Carbon and oxygen isotope composition of carbonate cement present δ13C values between −6.8 and −4.0 ‰ (mean −5.0 ‰) and δ18O values between −11.1 ‰ and −5.4 ‰ (mean −8.7 ‰). According to δ13C and δ18O calculations, ancient salinity Z values are distributed between 108.34 and 114.89 (mean 112.77), and the formation temperature of carbonate cement is between 43.36 and 77.84 °C (mean 62.30 °C). From the analysis of major elements, trace elements, and organic carbon, the diagenetic stage of the Upper Ganchaigou Formation is regarded as the B period of the early diagenesis stage or the A period of the middle diagenesis stage. The sedimentary environment is evolved from dry and cold freshwater and brackish water to warm and wet freshwater during deposition of the Upper Ganchaigou Formation.